The present invention relates to semiconductor devices and, more particularly, to methods of forming semiconductor devices.
Non-volatile memory devices are memory device that retain stored data even when power is not being supplied to the device. Phase change memory devices are one type of non-volatile memory device. A unit cell of a phase change memory device (hereinafter referred to as “phase change memory cell”) is the standard data storing element of a phase change memory device. Phase change memory cells have a stacked gate structure. Phase change materials have two stable states (i.e., an amorphous state and a crystalline state). The resistivity of a phase change material layer that has an amorphous state is higher than the resistivity of a phase change material layer that has a crystalline state. Using the resistivity difference resulting from states of a phase change material, data may be stored in, and read from, a phase change memory cell.
If a phase change material is rapidly cooled after being heated at a temperature higher than a melting temperature for a short time, it exhibits an amorphous state. On the other hand, if a phase change material is slowly cooled after being heated at a temperature higher than a crystallization temperature and lower than a melting temperature for a long time, it exhibits a crystalline state.
Conventionally, Joule heat is used to convert the state of the phase change material to an amorphous state or a crystalline state. The Joule heat is generated by flowing current through the phase change material and through a conductive plug which is in contact with the phase change material.
A phase change material requires a large amount of current to generate Joule heat for a program operation and/or an erase operation. As such, phase change memory devices may have relatively large power dissipation. Further, the plan area occupied by a switching element typically increases with increasing current. Accordingly, the relatively large current requirements may also increase the size of each phase change memory cell, making it more difficult to achieve high integration density.